In the manufacturing process of semiconductor integrated circuits said rotary coating apparatus is used to drop diffusion agent or photo resist on semiconductor wafers and form their uniform coatings thereon utilizing a centrifugal force.
In said rotary coating apparatus liquid to be applied is dropped around the center of the surface of a semiconductor wafer (hereinafter called a substrate) held by attraction on top of the disc of a spindle, with said spindle stopped, then said spindle is rotated to spread the liquid all over the substrate and at same time scatter unnecessary liquid out thereof due to the centrifugal force, thereby providing uniform diffusion agent or photo resist film over the surface of said substrate.
By the way, during the time required from just after forming a film over a substrate by spindle rotation to just before making dropping on the succeeding substrate, or between the dropping operations e.g. because of changes in applying conditions some solvent contained by liquid in contact with air will diffuse at the lower surface of said nozzle, causing a change in liquid viscosity.
The slightest change in said viscosity will exert a great influence on film thickness in the formation of coatings by means of spindle rotation; in the case of diffusion agent it will result in the dispersion of diffusion density; for photo resist, uneven film thickness will cause the internal distortion of exposure to photo resist, thus leading to inaccurate exposure and therefore it will become extremely difficult to obtain high-accuracy micropatterns.
Hitherto, any countermeasures have not been proposed to solve said problems.